Nickel-chromium-niobium alloy



United States Patent US. Cl. 75-171 Claims ABSTRACT OF THE DISCLOSURENickel-chromium alloys containing niobium, substantial amounts of carbonand, advantageously, molybdenum, wherein the niobium, carbon, chromiumand molybdenum constitutents of the alloys are carefully correlated toobtain good stress-rupture properties at 900 C. and higher.

This invention relates to nickel-base alloys capable of sustaining highstress at elevated temperatures for prolonged periods before fractureoccurs and, more particularly, to nickel-chromium alloys, containingboth niobium and carbon, which display these high strength properties.

. Most prior art nickel-base alloys depend for their hightemperaturestrength upon the formation of precipitates of phases containing nickeland also titanium or aluminum or both, commonly referred to as the gammaprime phase (gamma) represented by the formula Ni (Ti, Al). Other priorart alloys contain niobium and are strengthened, though lesseffectively, by a precipitated nickel niobide phase, and in yet others,phases of both kinds are present. In most of these alloys, however, thecarbon content is kept low, below 0.2% or even less, in order tominimize the formation of carbides of the addition elements at theexpense of the desired intermetallic phases. Such carbides areundesirable because in the temperature range in which the intermetallicphases are effective to harden and strengthen the alloys, carbides areless effective for this purpose.

As the temperature increases, the stress-rupture life of the alloys ofthe types above described decreases. This is due mainly to theinstability of the intermetallic phase which tends to coarsen atelevated temperatures, thereby becoming less efiective to strengthen thealloys, and eventually to dissolve and, to a lesser extent, to loss ofstrength by the matrix. At temperatures above 900 C., the stressrupturelife of even the strongest alloys hardened by inter- 1 metallic phasesfalls ofi rapidly.

Patented Sept. 9, 1969 It is an object of this invention to providenickel-base alloys which maintain high strength at temperatures of 900C. and above.

Another object of this invention is to provide nickelbase alloys which,when compared with nickel-base alloys hardened by precipitation of thegamma prime phase, do not appreciably harden on cooling from hightemperatures.

Generally speaking, the present invention contemplates alloys comprising(percent by weight) about 2% to about 6% niobium, about 0.3% to about0.5% carbon, about 19% to about 33% chromium, an amount of molybdenumsuch that:

(i) When the chromium content is less than 24% the molybdenum content isat least that given by the express1on percent Mo=% (24-percent Cr) (ii)When the chromium content does not exceed the molybdenum content is fromO to that given by the expression percent Mo=3.5 +0.5 (SO-percent Cr)and (iii) When the chromium content is greater than 30% the molybdenumcontent is from 0 to about 3.5%, up to about 0.01% boron, up to about0.1% zirconium, up to about 3% iron, up to about 2% cobalt, up to about2% silicon, up to about 1% manganese, and the balance being essentiallynickel with residual impurities and deoxidants, e.g., magnesium and/ orcalcium, in ordinary amounts which do not affect the basiccharacteristics of the alloys.

--Afurthe1' disadvantage of alloys containing titanium-..

and aluminum is that upon cooling from hot-working temperatures thealloys harden very rapidly. Hence, when the alloys are to be used in theform of sheet they must be quenched after hot working in order that thesheet may be soft enough to be conveniently fabricated by'cold forming.It is very difficult to quench the sheet material successfully withoutdistortion occurring, especially in large In accordance herewith we havefound that niobium contributes importantly to the stress-rupturestrength of the alloys. For adequate stress-rupture life at least 2.0%niobium is required, while preferably at least 2.5% is present and mostadvantageously at least 3.5%. Increasing the niobium content above themaximum of 6%, however, leads to a rapid drop in stress-rupturestrength, and the impact strength of the alloys also decreases somewhatas the niobium content increases.

Carbon is of great importance in the alloys, since it plays a large partin controlling the hardening and strengthening mechanisms. As the carboncontent is increased above 0.2%, the stress-rupture life rapidlyincreases, but at the same time the impact strength decreases and atcarbon contents above 0.5% it becomes inadequate.

To obtain the benefit of the strengthening effects of niobium andcarbon, the contents of chromium and molybdenum must be correlated.Chromium contributes to the stress-rupture strength of the alloys, andin the absence of molybdenum the alloys must contain at least 24%chromium. Chromium also contributes: to corrosion resistance, and if thehighest corrosion resistance is required the chromium content should behigh, i.e., at least 28%.

' On the other hand with increasing chromium contents the impactstrength decreases and if the highest impact strength is important, thechromium content should not exceed 25%. Alloys with high chromiumcontents are also diflicult to work, and if the chromium content exceeds33% they are unforgeable.

We have surprisingly found that although molybdenum does not improve thestress-rupture strength of alloys that are free from niobium, it is veryeffective in the niobium-containing alloys of the invention. When thechromium content is less than 24% it must be present, and at allchromium contents at least 1% molybdenum is preferred. The effects ofchromium and molybdenum are complementary, and as the chromium contentincreases the amount of molybdenum needed to achieve a givenstress-rupture strength decreases. Excessive amounts of molybdenum causethe stress-rupture strength to fall again, and the maximum amount thatmay be present decreases with increasing chromium content in accordancewith the formula percent Mo=3.5+0.5 (30percent Cr) until at chromiumcontents of 30% and above the molyb- Despite the relative insolubilityof the hardening phases at high temperatures, enough can be dissolvedfor substantial further precipitation to occur on subsequent aging at alower temperature and, in order to develop the stressrupture propertiesto the fullest extent, the alloys require dehhlh content h not exceedThe Optimum 5 heat treatment by solution-heating and aging. The higherStress-rupture Propertles are Obtained When the Chromium the temperatureof solution heating, the better the stressl molybdenum Contents areCorrelated in accordance rupture properties after aging. Advantageouslythe solu- Wlth the formula tion heating temperature is at least 1125 C.,e.g., 1150 Pement -p Cr) C. or even 1250 C., but it should not be sohigh that I contrast to alloys Strengthened by intermetauic incipientmelting occurs. A suitable period of heating is phases containingtitanium and aluminum, the stressfrom h' h to elght hours though longerPerlods may rupture properties of the alloys of the invention are not beused If deslredimproved by additions of boron or zirconium. If desired,The alloys may be coolhd from h solhhoh'heahhg however, th elements maybe present in amounts up to temperature at any convenient rate eitherdirectly to the 0.01% boron 1% Zirconium or both e'g" 0 0O5% agingtemperature or to alower temperature, conveniently boron and 0.03%zirconium. Nevertheless such additions room temperature, followed byreheating to the aging impair th ld bi i f the alloys, so boron and Ziptemperature. The rate of cooling from the solut1on-heato i are r f blnot added in making Sheet and ing temperature has l1ttle effect upon thehardness, and other fo where good Weldability is important even aftera1r-cool1ng to room temperature the hardness As to the otherconstituents, iron and cobalt impair the of the alloys 1n the form forample, of mch hlameter stress-rupture lives of the alloys. For thisreason the bar or Sheet is generally low enough to Permlt heavy amountthereof should not exceed 3% iron and 2% machining of h bar sgtock andcold'forming of h cobalt respectively and advantageously should not besheet; Q F 1f the hlghest stmss'rupthre l 15 above i it 1 1 Pr f bl bothare Substantially required t 13 advantageous to C001 the alloys rapldlyfrom absent, Sili i i Weldability and if good Weldability thesolution-heating temperature, e.g., by water-quenchis req ir d th iliContent Should not exceed If lng. Aging is suitably performed in therange of 750 C. weldability is not of importance, the silicon contentmay to 10600 C, -i C, for about 1 to 48 hourshe as hi h as 05% or even1% or 2% Up to 1% The duration of aging depends on the temperature, andganese may b present as an impurity. at 850 C. is conveniently 16 hours.

To facilitate hot working the alloys it is desirable that For thePurpose Of Y g those il n the art a they should nt i ll id l amounts fmagnesium ter understanding of the 1nvent1on, the results of numerous oral i up to Such amounts are tests carried out on alloys of differentcomposit1ons both monly re ent afte th use th r f as deoxidants in airwithin and without the invention are reproduced in several meltin nd ifVacuum-melting i employed it is also 35 tables shown below. As isreflected by these results, the advantageous t dd h b f Casting thealloy good stress-rupture propertles of the alloys of this inven- Anadvantageous combination of results is obtained hon depend on a delicatebalance among the alloylng when the alloys contain about 24% to about33% chroelements- The alloys used were Prepared y air-melting miurn ab t03% to about 05% carbon about to with a conventional addition ofmagnesium or calcium as about 6% niobium, abo t 1% t b t 35% 1 b adeoxidant, casting to ingots, forging to /8 inch diameter denum, up tobout 0 ()05% b up to about 3% bars, solution-heating for 2 hours at 1150C., air-cooling, Zirconium, up to about 0.3% silicon, up to about 1% andfinally aging for 16 hours at Suitable test manganese, up to about0,035% magnesium, up to about samples were thereafter machined from theaged bars. 0.035% calcium, and the balance essentially nickel. Astl'ess-l'uptum Properties Were dfilefmined at and particularlyadvantageous alloy has the composition 30% 'py impact Strengths Weredetermined at room Chromiurn, 5% niobium, 2.5% molybdenum, 0.4%carperature after soaking for 1,000 hours at 850 C. to reveal bon andthe balance essentially i k l any tendency of the alloys to embrittle inservice.

The hardening phases that form in th alloys f r Table I containsstress-rupture and impact data for invention remain largely undissolvedat temperatures at alloys containing y g amounts of niobium and y whichthe gamma phase dissolves in prior art nickeldenum with each alloynominally containing 30% chromium alloys hardened by titanium andaluminum. chromium and 0.4% carbon, the balance being nickel. Moreover,the hardening phases of our alloys are not Alloys Nos. 1 to 4 are alloysaccording to the invention completely dissolved even at temperaturesonly slightly whereas Alloy A, which contained no niobium, and alloysbelow the melting-point of the alloys. Thus, the alloys in B and C,which nominally contained 7% niobium, are not.

TABLE I Stress-rupture properties 3 t.s.i./900 c. 2 t.s.i./900 C.

Impact Nh Mo Life Elong. Life Elong. strength Alloy N 0. (percent)(percent) (hr.) (percent) (hr.) (percent) (it.lb.)

0 0 167 3 0 151 687 24 5 0 492 23 2,063 26 11 7 0 9s 51 2. 5 2. 5 2,14530 20 5 2. 5 s54 27 1, 063 0. 7 (i 7 2.5 86 23 1 Test discontinuedbecause of very low creep rate.

accordance herewith retain high strength at temperatures of 900 C. andabove. In addition, the alloys do not harden as rapidly and asextensively on cooling from high temperature as do the alloys hardenedby gamma phase.

Alloys A and B, in which the niobium contents were respectively lowerand higher than are required by the invention, had much inferiorstress-rupture lives in comparison with Alloys Nos. 1 and 2, and Alloy Cwas likewise much inferior to Alloys Nos. 3 and 4. Comparison of AlloyNo. 2 with Alloy No. 1 and of Alloy No. 4 with Alloy No. 3 alsoillustrates the concurrent increase in stress-rupture life and decreasein impact strength that results from increasing the niobium content.

The role of carbon is shown by the results reported It is essential thatthe alloy contains both niobium and carbon in order to formstrengthening phases rich in niobium.

Although no reliance is placed upon any particular theory to explain theinvention, it is believed that the in Table II relating to alloyscontaining 30% chromium 5 good stress-rupture properties possessed. bythe alloys of with varying amounts of carbon, niobium and molybourinvention at temperatures of 900 C. and above are denum, the balancebeing nickel. Alloys Nos. 1, 2, 3 and due to the presence in dispersedform of niobium car- 4, being examples of theinvention, are reproducedfor bide NbC, with or without the nickel niobide Ni Nb. The purposes ofthe comparison with Alloys D, E, F, G and 10 phase Cr C is frequentlypresent and may also cooperate H which are not examples of thisinvention since the carwith the niobium-rich phases to strengthen thealloys, bon content is either too low or too high. though it is not veryeffective alone at high temperatures.

TABLE II Stress-rupture properties 3 t.s.i./900 o. 2 t.s.i./900 0.

Impact C -Nb Mo Life Elong. Life Elong. strength Alloy N (percent)(percent) (percent) (hn) (percent) (hr.) (percent) (in-lb.)

1:. f. Q AlloyNo.

The importance of the presence of niobium-rich phases is shown by theresults in Table IV, in which niobium-free Alloys A, M, N and J arehardened by Cr C alone and have relative poor stress-rupture lives andare, therefore, not within the scope of this invention. In contrast,Alloy No. 2 contains 5% niobium and is strengthened by the precipitationof the niobide phases, NbC and Ni Nb, and.

is, therefore, an alloy within the teachings of this invention.

TABLE IV Stress-rupture prop erties, 2 t.s.i./900 C.

Nb Mo W Life Elong. Phases (percent) (percent) (percent) (hr.) (percent)present 5 0 0 2, 063 26 NbC, Ni Nb ClzgCn 0 0 0 167 ClzaCo 0 0 5. 0 18738 CmCu 0 2. 5 2. 5 479 CrzaCe 0 4. 9 0 118 24 CIzaCo (neither of whichis in accordance with the invention) shows the ineffectiveness of amolybdenum addition in the absence of niobium. The series of Alloys 2, 4and K and the series Nos. L, 6, 7 and8 show the elfects of increasingthe molybdenum content at chromium contents of 30% and respectively: allthese alloys are examples of the invention except Alloy K, whichcontains too much molybdenum with chromium, and Alloy L, which containsonly 20% chromium and no molybdenum. Each of these had poorstressrupture properties. The elfect of varying chromium alone is shownby comparison of wise much inferior to Alloys Nos. 3 and 4. Comparisonof Alloys 2, 5 and L.

It is worthy of mention that tungsten is not equivalent to molybdenum inthe alloys and in. addition is never beneficial and frequentlydetrimental to both the stress-rupture.

TABLE III Stress-rupture properties 3 t.s.i./900 C. 2 t.s.l./900 C.

Im act Cr Nb Mo Life Elong. Life Elong. strerigth Alloy No. (percent)(percent) (percent) (hr.) (percent) (hr.) (percent) (it.lb.)

TABLE V Stress-rupture properties, 3 t.s.l./900 c.

Nb Mo W Life Elong. strength Alloy No. (percent) (percent) (percent)(hr.) (percent) (it.lb.)

TABLE VI Stress-rupture properties, 3 t.s.i./900 C.

Nb Ta Mo Elong. Alloy No. (percent) (percent) (percent) Life (hr.)(percent) Tantalum should therefore not be added to the alloys,

though it may be tolerated as an impurity in the amounts commonly foundas an impurity in commercial sources of niobium, i.e. in amounts up toabout one-tenth of the weight of niobium.

The good corrosion-resistance of an alloy according to the invention isshown by the results of a test in which a specimen of Alloy No. 2 washalf-immersed for 16 hours in a molten bath of a mixture of 25% sodiumchloride and 75% sodium sulphate at 900 C. The loss in weight of thespecimen was only 14 mg./cm.

The advantageous properties of the alloys of the invention make themsuitable for use at elevated temperatures which may exceed 900 C., inthe form of sheet and components fabricated therefrom. They are alsouseful for articles and parts that require a combination of strength andresistance to corrosion at such temperatures, for example as parts ofheat-treatment furnaces, furnace belts, e.g., wire mesh belts, trays forarticles to be heat-treated, and the like.

We claim:

1. An alloy, age-hardenable by precipitation of at least oneniobium-rich phase, and consisting essentially of about 2% to about 6%niobium, about 0.3% to about 0.5% carbon, about 19% to about 33%chromium, and amount of molybdenum such that when the chromium contentis less than 24% the molybdenum content is at least (24-percent Cr)percent; when the chromium content does not exceed 30% the molybdenumcontent is from 0 to 3.5+0.5 (30percent Cr) percent; and when thechromium content is greater than 30% the molybdenum content is from 0 to3.5%, up to about 0.01% boron, up to about 0.1% zirconium, up to about3% iron, up to about 2% cobalt, up to about 2% silicon, up to about 1%manganese, and the balance essentially nickel.

2. The alloy set forth in claim 1 and containing at least 2.5 niobium.

3. The alloy set forth in claim 2 and containing at least about 1%molybdenum.

4. The alloy set forth in claim 3 in which boron does not exceed about0.005% and zirconium does not exceed 0.03%.

5. The alloy set forth in claim 4 in which silicon does not exceed 1%.

6. The alloy recited in claim 1 and containing about 24% to about 33%chromium, about 2% to about 6% niobium, about 1% to about 3.5%molybdenum and about 0.3% to about 0.5% carbon.

7. The alloy set forth in claim 6 in which the chromium content is atleast 28%.

8. The alloy set forth in claim 7 in which silicon does not exceed 1%.

9. The alloy set forth in claim 6 in which the niobium content is atleast about 3.5%.

10. The alloy set forth in claim '6 and containing about 30% chromium,about 5% niobium, about 0.4% carbon, about 2.5 molybdenum and thebalance essentially nickel.

References Cited UNITED STATES PATENTS 2,587,275 2/1952 Bash -1712,994,605 8/1961 Gill et a1 75l71 3,046,108 1/ 1962 Eiselstein 7517l3,069,258 12/1962 Haynes 75-171 3,046,108 7/ 1962 Eiselstein 75--171RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 148162 1313;" UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3iu6l7l DatedSeptember 9: 9 9

Inventor(s) Alfred John Fletcher and Edward Gordon Richards It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 5, line 60, delete entire line reading "wise much inferior toAlloys Nos. 3 and 1 Comparison of" Column 6, line 32, for "relative",read -relatively-.

Column 6, line 57, for "contain", read --contains-.

Column 7, line 50, for "and", read --a.n-.

sibwial mu, SEALED Mil-M12.

mm mun, I- molflm omniomotm

